U.S. patent number 4,422,806 [Application Number 06/235,194] was granted by the patent office on 1983-12-27 for sliding tension leg tower.
This patent grant is currently assigned to Chevron Research Company. Invention is credited to Barry J. Abbott, William H. Silcox.
United States Patent |
4,422,806 |
Abbott , et al. |
December 27, 1983 |
Sliding tension leg tower
Abstract
A compliant offshore drilling and producing structure is
disclosed. Axial piles extend from the sea floor above the water's
surface and are enveloped by sleeves extending downwardly from a
rigid platform. Buoyant chambers attached to the sleeves provide an
upward buoyant force in excess of the weight of the platform and
sleeves. This excess upward force is counterbalanced by the axial
piles through hydraulic means.
Inventors: |
Abbott; Barry J. (Concord,
CA), Silcox; William H. (San Francisco, CA) |
Assignee: |
Chevron Research Company (San
Francisco, CA)
|
Family
ID: |
22884492 |
Appl.
No.: |
06/235,194 |
Filed: |
February 17, 1981 |
Current U.S.
Class: |
405/227;
405/195.1; 405/223.1 |
Current CPC
Class: |
B63B
21/502 (20130101); E02B 17/00 (20130101) |
Current International
Class: |
E02B
17/00 (20060101); B63B 21/50 (20060101); B63B
21/00 (20060101); E02B 017/00 () |
Field of
Search: |
;405/195,196,200,203,224,227 ;114/264,265 ;175/7 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Corbin; David H.
Attorney, Agent or Firm: Newell; D. A. Keeling; E. J.
Claims
What is claimed is:
1. An offshore drilling and producing structure, which
comprises:
a rigid platform including equipment associated therewith;
a plurality of open-ended sleeves affixed to the platform and
extending downwardly therefrom for a substantial distance below the
water surface, in a substantially vertical orientation;
an equal plurality of axial piles secured to the sea floor which
extend upwardly into said open-ended sleeves to at least a position
near the surface of the water;
buoyant means affixed to said sleeves below the water line for
providing an upward buoyant force in excess of the weight of said
platform including equipment associated therewith and said
sleeves;
means for counterbalancing the excess buoyant force from the
plurality of axial piles said means permitting simultaneous
vertical movement of each of said sleeves with respect to each of
said piles to permit a desired degree of compliancy of rotation
about the sea floor.
2. An offshore drilling and producing structure as recited in claim
1, further comprising:
bearings situated between said axial piles and said sleeves to
facilitate the vertical movement of the sleeves with respect to
said piles.
3. An offshore drilling and producing structure as recited in claim
1, wherein at least 101% total weight of the structure, excluding
axial piles, is supported by the buoyant means.
4. An offshore drilling and producing structure as recited in claim
1, wherein at least 3 axial piles are used.
5. An offshore drilling and producing structure as recited in claim
1, wherein the length of the sleeves below the water surface
extends at least 75% of the water depth.
6. An offshore drilling and producing structure as recited in claim
1, wherein said means for counter balancing the excess buoyant
force and for permitting a desired degree of compliancy of rotation
about the sea floor includes:
at least one piston secured to the upper end of each of said axial
piles in a substantially vertical downwardly facing orientation
with respect to the piston axis;
a cylinder for each piston to travel which is secured to the
platform; and
means for injecting hydraulic fluid into said cylinders.
7. An offshore drilling and producing structure as recited in claim
6, wherein at least one cylinder connected from each axial pile is
connected to a single hydraulic circuit.
Description
FIELD OF THE INVENTION
This invention relates to offshore structures for drilling and
producing operations. In particular the invention is concerned with
a compliant structure suitable for use in water depths in excess of
1,000 feet.
PRIOR ART
The use of offshore structures for drilling and producing
operations has become relatively commonplace in recent years.
However, as more petroleum fields are being developed in deeper
waters, the search continues for structures capable of withstanding
the hostile wind and wave forces encountered without being
prohibitive in cost.
Three structures proposed in the prior art for operation in water
depths greater than 1,000 feet are the guyed tower, the tension leg
platform and the buoyant articulated tower. The guyed tower is a
trussed structure that is supported on the ocean floor with a spud
can or with pilings. Guy lines run from the deck to fairleads below
the water surface to clump weights on the ocean floor. Since the
tower will sway a few degrees during the passage of large waves,
the well conductors must flex at the tower base. Preferably the
fairleads are positioned at about the same elevation as the center
of pressure of the applied design wave and wind loads. The
environmental forces are therefore, more or less, colinear with the
mooring system and the moment transmitted to the tower base is
minimized. Beyond the clump weights, the guy lines are attached to
suitable fixed anchors. Thus, the clump wieghts may be lifted from
the bottom by heavy storm waves permitting further displacement of
the tower.
An articulated buoyant tower differs from the foregoing fixed
structure in several important respects. An articulated joint, such
as a universal or ball joint, attaches the tower to a pile base
thereby permitting the tower to tilt in response to environmental
forces. A set of buoyant chambers provide the necessary righting
moment and the upward force is effectively negated by a ballast
chamber located near the bottom of the tower. The primary objection
to such articulated systems arises as a result of the tower's lack
of redundancy and the difficulty of inspection and/or replacement
of the articulated joint.
A tension leg platform is a buoyant floating structure held in
place by vertical tension cables anchored to the sea floor. The
flotation chambers are designed to minimize the platform's response
to weather and wave conditions.
The present invention combines the better features of the above
systems in a new and ingenious manner to produce a superior
structure for offshore drilling and producing operations.
SUMMARY OF THE INVENTION
The present invention relates to a compliant offshore drilling and
producing structure. In accordance with the invention a plurality
of axial load piles installed in the sea floor extend upwardly
therefrom to a point beyond the upper surface of the water. A rigid
platform is provided having a plurality of open ended sleeves
affixed thereto and extending downwardly therefrom in a
substantially vertical orientation over each of the axial piles.
Buoyant means affixed to the sleeves below the water line are used
to provide a buoyant upward force in excess of the weight of the
platform, equipment and sleeves. Means are also provided for
counterbalancing the buoyant forces in excess of the platform
weight from the plurality of axial load piles. Preferably these
latter means comprise pistons attached to the ends of the axial
piles which extend downwardly into hydraulic cylinders secured to
the platform. Means are provided for injecting hydraulic fluid into
each of the cylinders and preferably groups of the cylinders are
connected to a single hydraulic circuit.
Bearings are provided between the axial piles and the sleeves to
facilitate vertical movement of the sleeves and platform relative
to the fixed axial piles. The buoyant chambers should be
compartmented to prevent a compressive load from being applied to
the axial piles in the event of a rupture in the chambers. If
required, skirt piles may also be installed near the base of the
structure to provide additional lateral support.
BRIEF DESCRIPTION OF THE DRAWING
The drawing is a schematic diagram of apparatus suitable for use in
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawing there is shown a structure in accordance
with the present invention, generally referred to by reference
numeral 10. A plurality of axial load piles 12, preferably at least
3 in number, are driven into the sea floor 14 to a suitable depth
to provide an adequate resistance against the environmental forces,
primarily wind and wave, which may occur. As illustrated, the piles
extend upwardly from the sea floor beyond the water's surface
16.
A platform 18 which provides the necessary working space for the
drilling and producing operations and which may also provide
housing and office space for the crew is situated above the water
line beyond the height of the maximum anticipated storm sea.
A plurality of sleeves 20 are rigidly attached in any conventional
manner to the platform 18 and extend vertically downward over each
of the axial piles. Preferably, the sleeves will extend below the
water line at least 75% of the distance to the sea floor. The
sleeves are also preferably cross braced with stiffening trusses 22
substantially along their underwater lengths.
Bearings 24 are provided between the sleeves 20 and the piles 12 to
facilitate relative axial movement therebetween. The bearings may
be of any suitable and conventional design to lower the frictional
forces which would otherwise develop and provide lateral support to
the axial piles. Under the conditions of use, the bearings should
preferably be designed as a permanent system which will not require
replacement during the life of the structure. Where this is not
possible, sufficient access should be provided to the components to
the bearing system so that it is possible to replace critical
elements with minimum dismantling of adjacent components.
Preferably 101-105% of the weight of the entire structure,
including the platform and its associated equipment, and excluding
the shear piles, will be supported by buoyancy chambers 26
conventionally affixed to the sleeves beneath the water line.
Buoyancy chambers 26 provide a righting moment to the tower
whenever it sways from a true vertical orientation due to
environmental forces. These chambers should be compartmented so
that unexpected sealing failures will not unduly burden the
foundation pilings.
Normally two sets of buoyant chambers will be used for the
structure's tow and installation at the drilling site. The chambers
provided for supporting the lower portion of the sleeves during
transportation may be flooded to submerge the structure, removed,
or shifted towards the upper end of the unit.
The upper end of each axial pile extends through its associated
sleeve as shown in the drawing and is connected by cross arms 28 to
pistons 30. Each piston is housed in a hydraulic cylinder 32
affixed to the platform in a load bearing relationship. Preferably
at least one cylinder attached to each axial pile is serviced with
hydraulic fluid via lines from a single fluid reservoir housed in
the platform. As shown in the drawing, line 34 provides a flow path
for hydraulic fluid from reservoir 36 to the outer cylinders and
line 38 provides a flow path for hydraulic fluid from reservoir 40
to the inner cylinders.
The excess buoyant force over the weight of the platform and
sleeves is counterbalanced by tension in the axial piling through
the hydraulic cylinders, fluid and pistons. This system gives the
overall structure the desired degree of compliancy of rotation
about the sea floor, but resists platform heave or vertical
motion.
To provide additional lateral support, skirt piles 42 may be
installed in the sea floor near the base of the platform.
Vertically slidable sleeves 44 transmit lateral loads from the
skirt piles through a truss 46 rigidly affixed to sleeves 20.
Bearings 48 may be inserted between the skirt piles 42 and sleeves
44 to facilitate relative axial movement.
While use of the hydraulic means as set forth above is preferred
for coupling the structure sleeves and platform to the axial load
piles, it is within the spirit and skill of this invention to use
conventional mechanical systems to accomplish the same end.
* * * * *